The present invention relates generally to the field of fiber spinning methods and apparatus, especially the melt-spinning of fibers from thermoplastic materials. In preferred forms, the present invention relates to the continuous direct spinning of anionically polymerized caprolactam, and to apparatus for accomplishing the same.
The melt-spinning of thermoplastic materials to form essentially endless-length fibers is notoriously well known. Typically, the desired thermoplastic material from which fiber is to be melt-spun is polymerized from suitable monomers to form a solid reaction product. The resulting solid thermoplastic material is then formed into chips, washed and dried. The dried thermoplastic chips are then transported physically to the inlet of an extruder and re-melted. The melted thermoplastic material is discharged from the extruder and fed to a spinneret having numerous orifices shaped and sized to form the desired fiber cross-section. The molten thermoplastic material is thus extruded through the orifices of the spinneret to form a yarn composed of numerous melt-spun fibers. The extruded fibers are allowed to cool downstream of the spinneret and are then typically collected on a winder.
The anionic polymerization of monomeric lactams, especially caprolactam, to form polyamides is also well known. (See, for example, U.S. Pat. No. 3,342,784, the entire content of which is expressly incorporated hereinto by reference.) Such anionically polymerized lactams represent an attractive alternative to the conventional melt-spinning technique of re-melting solid thermoplastic materials and then extruding the melt through orifices of a spinneret. That is, the anionic polymerization of lactams exhibits faster reaction kinetics and requires fewer process steps as opposed to the more conventional hydrolytic route to the polymerization of lactams.
There are several problems inherent in the anionic polymerization of lactams due, for example, to the more complex chemistry involved as compared to the more conventional hydrolytic polymerization route. Specifically, the anionic polymerization of lactams usually involves both an initiator and a co-initiator to be present in the reaction mixture with the lactam monomer. The reaction is much more difficult to control due to the more sensitive temperature vs. time profiles that are needed. As a result, it is much more difficult to maintain consistent polymer properties and/or to include uniform distribution of additives in the polymer. Furthermore, the reaction streams must be mixed rapidly due to the rapid reaction rate to produce consistent product. Heat must be applied quickly and uniformly because the reaction is rapid and temperature-sensitive.
When one attempts to directly spin filaments of anionically polymerized caprolactam, these quick reaction times and temperatures are even further exacerbated due to the relative small size of the object being formed (i.e., the filament). Thus, it is critical that effective heat and mass transfer be present in order for the rapid production of uniform polymer for relatively small objects, such as melt-spun filaments where inconsistencies in molecular weight will produce denier variation, variation in dyeing and weaker filaments. It is therefore towards solving such problems associated with rapid and/or temperature sensitive reactions, such as the anionic polymerization of lactams, and their direct spinning into fiber that the present invention is directed.
Broadly, the present invention relates to processes and systems by which a stream of polymerization reactants having a relatively rapid reaction completion time may be polymerized continuously. In accordance with one aspect of the present invention, the polymerization reaction stream is continuously fed along a flow path sequentially through a mixer and a reactor zone having multiple substantially thermally-separated reactor sections along the stream""s flow path. A reaction mixture is thus formed by rapidly mixing the reaction stream within the mixer on a time scale that is small in comparison to the reaction completion time of the polymerization reaction stream. Heat is uniformly applied across the reaction mixture within at least one of the reactor sections on a time scale that is small in comparison to the reaction completion time of the polymerization reaction stream.
In some specific preferred embodiments of the present invention, therefore, processes and apparatus are provided by which polymerization reactions which are rapid and/or temperature-sensitive can be effectively conducted and controlled. That is, according to the present invention, miniature reactors are employed having both effective mass transfer (resulting in good mixing of the reactants) and effective heat transfer (resulting in temperature equalization) on a time scale that is sufficiently small compared to the time needed for the reaction to proceed and on a space scale that is matched to the objects being produced directly from the polymer. In especially preferred embodiments, the present invention relates to the anionic polymerization of caprolactam using sufficiently miniaturized reactors and the direct spinning of the resulting polycaprolactam into fiber (i.e., without intervening solidification of the polycaprolactam) of commercially acceptable quality.
One exemplary useful embodiment of the present invention involves the continuous anionic polymerization and melt-spinning of a polycaprolactam by forming a reaction mixture from at least two streams of liquid caprolactam respectively containing a polymerization initiator and co-initiator, and then subjecting the reaction mixture to anionic polymerization reaction conditions to obtain a molten polycaprolactam. The molten polycaprolactam is then directly (i.e., without intermediate solidification) transferred to, and extruded through, a fiber-forming orifice of a spinneret to form a fiber thereof.
An exemplary system to achieve such continuous anionic polymerization and melt-spinning of polycaprolactam includes a mixer for receiving and mixing at least two streams of liquid caprolactam respectively containing a polymerization initiator and co-initiator, and a reactor and melt-spinning apparatus downstream of the mixer. The reactor and melt-spinning apparatus most preferably is formed of multiple plates which establish a reactor zone having a flow path for the reaction mixture which is sized and configured to achieve a predetermined residence time of the reaction mixture therein, and a temperature control zone in thermal communication with the reactor plate for controlling the temperature of the reaction plate to achieve anionic polymerization reaction conditions while the reaction mixture is flowing along the defined flow path therein to obtain the molten polycaprolactam. A spinneret zone downstream of the reactor zone thus receives the molten polycaprolactam directly and forms a fiber therefrom by extruding it through the spinneret""s fiber-spinning orifice.
These and other aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.